You can just add it to your Steam library even if it's DRM-free, menu-bar "games"-->add a Non-Steam game to your library... the board here on Steam Community is exactly where we are and be aware that this way the Steam overlay with every function works just fine with the GOG DRM-free game.

You can just add it to your Steam library even if it's DRM-free, menu-bar "games"-->add a Non-Steam game to your library... the board here on Steam Community is exactly where we are and be aware that this way the Steam overlay with every function works just fine with the GOG DRM-free game.